A STUDY OF RECENT EARTHQUAKES. 823 



responds, without doubt, with the deep-seated center of impulsion, 

 and has been called the epicenter. Here is where the vertical shocks 

 or succussions are most usually felt. From these radiate the undula- 

 tory movements, the speed of propagation of which has been estimated 

 at from 1,100 to 1,500 feet per second, or about that of sound in the 

 air. Sometimes the area of disturbance is very limited, even when 

 the convulsion is most violent ; at other times it is very extensive, as 

 was the case in the Lisbon earthquake of 1755, which affected a sur- 

 face equivalent to one-thirteenth of that of the globe. Most frequently 

 the linear dimensions of the agitated surfaces are included within from 

 65 to 600 miles, or from one four-hundredth to one fortieth of the cir- 

 cumference of the earth. 



The area of disturbance is usually irregular in form, and not cir- 

 cular, as was once supposed. Sometimes it is much lengthened in one 

 direction, following the lines of neighboring mountains or other im- 

 portant accidents of structure. During the earthquakes of 1783 and 

 October, 1876, the chain of the Apennines served as a protecting wall 

 to the eastern provinces of the Italian Peninsula. While one side of 

 the chain was assailed by thousands of shocks, which caused great dis- 

 asters, nothing whatever was felt on the opposite slope. In the Alps, 

 most of the earthquakes take place in the lateral, north and south 

 spurs, which are formed of sedimentary beds, while the central chain, 

 composed of crystalline rocks, is not disturbed. The Andes of South 

 America form a natural bulwark which the strongest convulsions of 

 the Pacific littoral, while they extend a long distance parallel to the 

 chain, hardly ever cross ; and, if occasionally a few shocks are propa- 

 gated beyond it, they become extremely weak. 



The movements are very unequally perceptible within the area of 

 disturbances ; and between two points shaken by the same impulsion 

 there may be intermediate points that continue quiet. These are some- 

 times called bridges or arches. The shocks are frequently accompa- 

 nied by noises resembling heavily loaded wagons rolling over the 

 pavement, or subterranean thunders or roarings ; but their intensity 

 bears no kind of proportion to that of the agitation. The great earth- 

 quake of Riobamba, in 1797, was silent. But the sounds have relation 

 to the rocks that transmit them. Then there are subterranean rum- 

 blings that are associated with shocks like the bramidos of Guanajuato, 

 in Mexico, in 1784, continuing for a month, under terror of which the 

 inhabitants left the city. There were flashes as of lightning, alternat- 

 ing with long rollings, like that of distant thunder. This phenomenon 

 gradually passed away. The noises associated with earthquakes seem 

 to be of the same nature as those that accompany eruptions. The lat- 

 ter are propagated through the ground, not the air, for hundreds of 

 miles. But nothing else that is known of this kind reaches the pro- 

 portions of what took place on the 26th of August, 1883, at the erup- 

 tion of Krakatoa, the sounds of which were heard within the whole 



